Nuclear Magnetic Resonance (NMR) refers to a physical response of nuclei to a magnetic field. NMR is one of the most common techniques used today to examine the content of a material. One application of NMR techniques is in the evaluation of an earth formation during formation drilling and logging operations. In operation, today's standard logging NMR tool produces a static magnetic field and directs this field into an earth formation of interest. Researchers have recognized that some particles of an earth formation such as atomic nuclei and protons have magnetic spins that tend to align with a static magnetic field B.sub.0 imposed on a formation. The tool produces this magnetic field with one or more magnets. By configuring the combined magnetic fields of a set of magnets in the NMR tool and then directing the magnetic field into the formation, hydrogen nuclei in the formation align with the magnetic field generated by the magnets. When the magnetic field is removed, the nuclei return to their original orientation giving off a radio-frequency signal. Tool equipment records the amplitude of the radio-frequency signal as the free-fluid index, which is an indication of the amount of water or hydrocarbons that are not bound to any surface in the formation
Basic Principles of Nuclear Magnetic Resonance
The hydrogen atom is of primary importance in NMR techniques because of its high natural occurrence in hydrocarbons. The hydrogen atom consists of a single proton (the nucleus) and an electron orbiting around the proton. In the classical picture, the proton rotates (spins) around its axis; this rotation results in angular momentum Ih.sup.l, where I is called the spin and h.sup.l is Planck's constant h divided by 2.pi.. The angular momentum is a measure of angular motion expressed by the product of the momentum of inertia of the nucleus and the angular velocity .omega..sub.rot. The rotation (spinning) generates a magnetic moment .mu.. In the absence of an external magnetic field, the individual magnetic moment in an ensemble of hydrogen nuclei (protons) are randomly oriented. However, in an externally applied magnetic field B.sub.0, the magnetic moments of the protons are forced to be aligned so that the net magnetization (total magnetic moment/volume) is either parallel or anti-parallel with the direction of the field. The magnetic moments of the individual protons precess around the axis B.sub.0 at a specific angular frequency .omega..sub.0 (Larmor frequency). The Larmor frequency is determined by the magnitude of B.sub.0 according to the equation .omega..sub.0 =.gamma.B.sub.0, where .gamma. is the gyromagnetic ratio which is related to .mu. by .gamma.=.mu./h.sup.l I=42.58 MHz/T for protons. (The Larmor frequency is the frequency at which gyromagnetic moments precess in a magnetic field. Atoms and nuclei posses magnetic moments because of their spin and precess like small gyroscopes about the direction of an externally applied steady magnetic field (such as the earth's field). Radio-frequency energy at right angles to the steady field will be absorbed because of resonance when the RF-frequency equals the precession frequency.) At thermal equilibrium, the number N.sub.p of protons whose z component of magnetic moment is oriented parallel to the direction of the magnetic field is slightly greater than the number N.sub.a of antiparallel protons, depending on B.sub.0 and the sample temperature T. The slight preponderance of parallel protons results in a net nuclear macroscopic magnetization M.sub.0 which is the resultant of the individual magnetic moments in the sample, along the direction of B.sub.0.
In order to detect M.sub.0 it is necessary to irradiate the formation with an RF magnetic field B.sub.1 at approximately the same frequency as the Larmor precession frequency .omega..sub.0, but applied perpendicular to B.sub.0 by an angle .theta.. If the applied ratio frequency (RF) magnetic field B.sub.1 is turned off (thereby forming a pulse), the magnetization M.sub.0 precesses around the direction of B.sub.0. According to Faraday's induction law, the precessing magnetization M.sub.0 induces a voltage in a coil wound with its axis in the precess plane and tuned to the Larmor frequency .omega..sub.0. The voltage induced in the coil is detected as the NMR signal. The hydrogen nuclei (protons) of water and hydrocarbons occurring in rock pores produce NMR signals that are distinct from any signals induced in other rock constituents. A population of such nuclei having a net magnetization, tends to align with any imposed field such as B.sub.E.
Earlier NMR tool's, such as Schlumberger's NML.TM. nuclear magnetic logging tool (See U.S. Pat. No. 4,035,718 issued to Richard N. Chandler) uses the earth's magnetic field and at least one multi-turn coil wound on a core of a non-magnetic material. The coil is coupled to the electronic circuitry of the tool and is designed to periodically apply a strong DC polarizing magnetic field, B.sub.P, to the formation in order to align spins approximately perpendicular to the earth's field, B.sub.E. The characteristic time constant for the exponential buildup of this spin polarization is called the spin-lattice relaxation time, T.sub.1. At the end of polarization, the field is rapidly terminated. Since the spins are unable to follow this sudden change, they are left aligned perpendicular to B.sub.E and therefore precess about the earth's field at the Larmor frequency f.sub.L =.gamma.B.sub.E, where .gamma. is the gyromagnetic ratio of the proton. The Larmor frequency in the earth's magnetic field varies from approximately 1300 to 2600 Hz, depending on location. The spin precession induces in the coil a sinusoidal signal of frequency f.sub.L whose amplitude is proportional to the number of protons present in the formation.
The NMR tool is usually deployed in a borehole and is surrounded by borehole fluid. This fluid material also generates a signal that can affect the tool's measurements. Because the resonance region of an electromagnetic signal extends into the borehole, the signal and more specifically a magnetic resonance signal is produced in the borehole fluid. Such a magnetic resonance signal from the borehole fluid is detected along with the desired NMR signal. This borehole signal must be eliminated or reduced because the NMR device functions by detecting protons in fluids. Typically, the rock formation is 0-30% fluid by volume, but the borehole fluid contains more than 50% fluid which has a high density of hydrogen nuclei. For this reason, the magnetic resonance signal of the borehole fluid would dominate any formation signal detected by the pulsed NMR device. One solution to borehole interference is to add fluid additives, such as magnetite, to the borehole fluid to suppress the borehole fluid signal, thereby preventing the fluid signal from obscuring the formation signal. However, the logistic difficulty of doping the mud contributed to the infrequent use of the earlier NMR logging tool.
Another obstacle in NMR logging is the shape of the borehole wall. As shown in FIG. 1, in an ideal logging situation, the borehole wall 1 would have a uniform and straight shape. This uniform borehole wall would enable a nuclear magnetic resonance tool 2 to be positioned in close proximity with the formation 3 surrounding the borehole and there would be minimal tool standoff 4. A uniform borehole 5 also reduces the effect that the borehole signal has on the actual measurement. However, in an actual logging situation, the borehole wall 1 can have the shape shown in FIG. 2. In this type of borehole, tool standoff 4 and borehole effects are greatly increased by the borehole wall shape.
While the early generation of NMR tools have the capability to extract information about the earth formation and fluid properties, these tools and techniques have some disadvantages which limit their utility in practical applications. With the newer NMR tools, because of the requirement of the application of the RF field, the precession frequency is fixed, and the depth of investigation from the borehole into the formation of the nuclear magnetic properties is restricted to a shallow region of the formation around the borehole wall, approximately 1 to 3 inches. Therefore, there is a need for a nuclear magnetic resonance system and method that will extend the depth of investigation into the formation of the NMR measurement.